Harry, do you pay the prospective employees during the trial run? The blog post implies that it's a week long, ("We’ve been able to spot problems during the week..."). While the information gained by both parties during a week of working together is no doubt very valuable for them both, a week is a long time to work for no pay.

EDIT: Oh, I see your edit, these trials runs typically aren't a full week long. But in any case, if someone is going to give up a substantial amount of time, I hope you pay them for it.